Modern machinery for processing running webs into paper products, such as tissue and paper toweling, employs a vacuum applied to the running web to adhere the running web to a moving carrier fabric. Several carrier fabrics may coordinate together, and each carrier fabric typically moves in an endless loop. The running web is transferred from one carrier fabric to another carrier fabric during the papermaking process, beginning at a headbox slurry and ending with a finished product of one or more layers.
Historically, flat carrier fabrics have been used to manufacture such products. However, in many modern uncreped through air dried (xe2x80x9cUCTADxe2x80x9d) tissue making processes, some of the carrier fabrics used are not flat, and include a raised pattern projecting from the fabric surface that includes topographical features.
It is a significant manufacturing challenge to reliably and securely transfer a running web when the fabric employed to carry the running web is not flat. Flat carrier fabrics tend to hold tightly the running web with a minimal amount of vacuum applied to the running web, while a carrier fabric employing topographical features typically requires a greater degree of vacuum pressure to reliably hold the running web in place upon the surface of the carrier fabric. Thus, a need exists to provide improved methods and apparatus for transferring running webs from one carrier fabric to another carrier fabric, especially for carrier fabrics that employ topographical features.
The amount of vacuum that must be applied to tightly adhere a running web to a moving carrier fabric varies. The vacuum must be great enough to ensure reliable transfer, but not so great as to damage the product. For example, if the level of vacuum is too high, undesirable holes or defects may be formed in the running web. Thus, one significant challenge in the papermaking industry is to regulate the level of vacuum applied to a carrier fabric to ensure a reliable manufacturing process that produces a high quality product.
The apparatus used to transfer a running web from one moving carrier fabric to another is commonly called a xe2x80x9cpick up boxxe2x80x9d, or xe2x80x9cbox.xe2x80x9d The reservoir or manifold connected to the xe2x80x9cboxxe2x80x9d employing a vacuum is sometimes called a xe2x80x9cvacuum box.xe2x80x9d Vacuum boxes typically have been oriented across the entire width (i.e. cross direction) of a running web. Pick up boxes apply a vacuum force to rapidly and securely pull a running web from a first fabric to a second fabric.
Unfortunately, web transfer difficulty is sometimes experienced near the edges of the running web. That is, the medial portion of the running web may transfer well, but the edges may not transfer reliably to the receiving carrier fabric, which can be a significant problem. This problem may be especially pronounced at low transfer vacuum levels. In some applications, the problem may occur in part because web edges are not smooth and flat during transfer. Poor edge transfer sometimes causes the running web to strike downstream equipment, such as a through air dryer, causing a web xe2x80x9cpile-upxe2x80x9d, which may result in undesirable production problems.
What is needed in the industry is an apparatus, system and method for transferring a running web among several carrier fabrics in a secure and reliable manner, while avoiding or minimizing damage to the final manufactured product. A transfer that secures the edge of the running web to the receiving carrier fabric without damaging equipment or causing production problems would be highly desirable. An apparatus or method that is capable of performing the transfer at relatively low vacuum levels would be especially desirable.
A vacuum apparatus for a papermaking machine is provided in one embodiment. The papermaking machine provides at least a first and second carrier fabric, said first and second carrier fabrics each having a machine direction and a cross direction perpendicular to the machine direction. In combination, a running web is provided for travel on the apparatus, the running web being adapted to proceed in the machine direction. The running web includes a width in the cross direction, the running web being bounded by a first edge and a second edge. In one aspect of the invention, a primary head is positioned adjacent to and in fluid communication with the running web. A primary vacuum box may be joined to the primary head, the primary vacuum box having on its interior a first region of reduced air pressure. The primary vacuum box may extend in the cross direction and can be adapted for applying reduced air pressure to the primary head for application to the running web in transferring the web from the first carrier fabric to the second carrier fabric.
A first auxiliary head also may be positioned adjacent to and in fluid communication with the first edge of the running web. Furthermore, an auxiliary vacuum box is joined to said first auxiliary head, the auxiliary vacuum box having an auxiliary region of reduced air pressure, the auxiliary vacuum box extending in the cross direction. The auxiliary vacuum box is adapted for applying reduced air pressure to the first auxiliary head for application to at least the first edge of the running web.
In another aspect of the invention, a second auxiliary head is provided and is adapted for applying reduced air pressure. In some applications, the first auxiliary head is applied to a first edge of the running or carrier fabric, while the second auxiliary head is applied to a second edge of the carrier fabric.
In some embodiments of the invention, there may be one or more auxiliary heads and/or vacuum boxes, without a primary vacuum box. That is, auxiliary vacuum boxes may be applied in essentially any location where edges of a running web must be stabilized against a carrier fabric.
A system also is provided for transferring a running web from a first carrier fabric to a second carrier fabric. The system includes a running web having a cross direction extending from a first edge to a second edge. The running web is configured for traveling in a machine direction from an upstream end of the manufacturing process to the downstream end of the process. A primary head is positioned adjacent to and in fluid communication with the running web. The primary head includes a primary contact means for application to the running web, the primary contact means extending in the cross direction of the running web. The primary contact means forms a vacuum slot adjacent to the running web.
A primary vacuum box is joined to the primary head, the primary vacuum box having an interior region of reduced air pressure. The primary vacuum box is adapted for applying reduced air pressure to the vacuum slot of the primary contact means for application to the running web to transfer the running web from the first carrier fabric to the second carrier fabric. The first auxiliary head may be positioned downstream from the primary head. The first auxiliary head includes a first auxiliary contact means extending in the cross direction of the running web and in contact with the running web. The first auxiliary head typically is provided adjacent to and in fluid communication with the first edge of the running web. A first auxiliary vacuum box is joined to the first auxiliary head. The first auxiliary vacuum box includes an auxiliary region of reduced air pressure. The auxiliary vacuum box is adapted for applying reduced air pressure to the auxiliary contact means for application to the first edge of the running web. In some applications, the auxiliary contact means comprises two lips having a vacuum slot between the lips. In such applications, the lips are applied to the surface of the carrier fabric to pull a vacuum through the fabric, thereby adhering the running web to the carrier fabric.
A process also is provided for manufacturing paper from a web in a papermaking machine. The process includes applying a first suction force at a first point on the web, and a second suction force at a second point which is downstream from the first suction force. These forces, acting together, transfer the web from the first carrier fabric to a second carrier fabric. The second suction force is applied to the first and second edges of the web, thereby completing transfer the web from a first carrier fabric to a second carrier fabric.